Calcium-strontium-lead grid alloy for use in lead-acid batteries

ABSTRACT

A novel calcium-strontium-tin-lead grid alloy is disclosed which exhibits a finer grain structure, higher hardness, improved castability and superior corrosion resistance.

BACKGROUND OF THE INVENTION

For many years lead-acid batteries, and particularly lead-acidautomotive batteries have incorporated battery grids cast oflead-antimony grid alloys. These lead-antimony alloys exhibit suitablehardness and resistance to corrosion, however have the undesirable sideeffect of encouraging "local action", sulphation and gassing that is,electrolysis of the sulfuric acid solution during overcharging.

More recently with the development of what has been termed "maintenancefree" batteries -- batteries which are not intended to have water addedto them, or to which water is to be in-frequently added -- practitionersin the art have begun to look for other materials which exhibit suitablestrength, castability, hardness, and corrossion resistance, whileeliminating the gassing and other problems heretofore exhibited by highantimony grid alloys.

For many years it has been known to utilize either strontium or calcium,with or without tin, in lead alloys used to form the grids of storagebatteries.

In U.S. Pat No. 2,040,078 entitled, "Lead Alloy", dated May 12, 1936strontium is disclosed as imparting a desirable degree of hardness tolead, while overcoming certain disadvantages previously associated withlead-antimony alloys. For instance, 0.08% strontium added to pure leadis disclosed as providing an alloy which, after quenching from 300° C.and aging for 3 weeks at 100° C., has a Brinell hardness from 7 to 9,which is a desirable hardness for storage battery grids. Alternatively,U.S. Pat. No. 2,040,078 discloses the use of strontium in combinationwith tellurium.

Also of interest to the present application is U.S. Pat. No. 2,013,487entitled, "Lead Alloy", dated Sept. 3, 1935 wherein it is disclosed thatgood results were obtained with alloys containing from 0.2% to 1.5% ofone of the elements chosen from the group of barium, strontium, andcalcium; tin from 0.2% to 2.0%; and with the remainder being lead. Inparticular, U.S. Pat. No. 2,013,487 additionally discloses that any twoor all three of the metals barium, calcium and strontium may be used inthe same alloy "providing the total percentage thereof lies between 0.2%and 1.5%, the quantity of tin being as above mentioned". (allpercentages being taken by weight).

SUMMARY OF THE INVENTION

The present invention provides a novel calcium-strontium-tin grid alloyfor use in lead-acid batteries wherein the total weight percentage ofcalcium and strontium is far less than that heretofore through necessaryto produce suitable storage battery grids. This fact results from thediscovery that calcium and strontium, particularly when used incombination with tin, exhibit an unexpected synergism when combined insuch an alloy. In particular, it has been found that a given weight ofapproximately equal amounts of calcium and strontium produces a farsuperior lead alloy to that produced when equivalent weights of eithercalcium or strontium are combined with lead. Accordingly, it has beenfound that a desirable battery grid alloy may be produced wherein verylittle calcium and/or strontium need be added in order to produce asuitable lead grid-alloy. For example, a suitable grid may comprise atotal calcium-strontium weight percent of only about 0.08 wt-% yetexhibit characteristics similar to those exhibited only by grids havingseveral times this amount of either strontium or calcium. While theparticular interaction between these elements is not fully understood,it is known that the relatively large grain structures of eithercalcium-tin-lead or strontium-tin-lead alloys are not exhibited by thecalcium-strontium-tin-lead alloy of the present invention. Rather thegrain structure becomes proportionally refined as the proportions ofcalcium to strontium approach " 50-50", such refined grain structurebeing accompanied by increasing hardness. In addition to the economy ofusing less calcium or strontium to harden the grid alloy, the presenceof less of these materials in the alloy and the novel interactionbetween these materials at these levels minimizes the local action,gassing, corrosion and/or sulphation of the grids formed thereof.

Accordingly, the preferred embodiment alloy of the present inventionconsists essentially of 0.01 to 0.09 wt-% calcium, 0.01 to 0.10 wt-%strontium, and 0.1 to 1.0 wt-% tin, with the remainder of the alloybeing lead and standard trace impurities. As mentioned above, in thepreferred embodiment, the amounts of calcium and strontium used in thepreferred grid alloy should be approximately equal to achieve maximumhardness. It is preferred to keep the amounts of calcium and strontiumat levels of about 0.03 to 0.04 wt-% calcium and about 0.04 to 0.05 wt-%strontium, with tin in the preferred range of from about 0.3 to 0.5wt-%.

In alternate embodiments, trace amounts of aluminum in weightpercentages less than 0.03 wt-% and preferably less than about 0.02 wt-%may be utilized to inhibit oxidation, and alternatively may alsoincorporate, in combination with aluminum, trace amounts of lithuim,cadmium, or less preferably, magnesium, provided the total of thesetrace elements does not exceed the aforementioned percentages.

Accordingly, the primary object of the present invention is theprovision of a superior lead-alloy for use in casting the grids oflead-acid storage batteries. A further object of the present inventionis the provision of a fine grain structure, high hardness, easilycastable grid alloy exhibiting superior corrosion resistance and minimal"local action" or "gassing" when incorprorated as part of a lead-acidbattery. These and other objects of the present invention will becomeapparent from the following examples:

Although specific forms of the invention have been selected forillustration and the following description is drawn in specific termsfor the purpose of describing these forms of the invention, thisdescription is not intended to limit the scope of the invention which isdefined in the appended claims.

EXAMPLE I

One preferred grid alloy made in accordance with the present inventionwhich has been found to be particularly suitable for use in forming thegrids of lead-acid storage batteries consist essentially of: 0.04 wt-%strontium; 0.3 wt-% tin; trace impurities not exceeding about 0.02 wt-%,as for example, aluminum, cadmium, and lithium; with the remainder beinglead.

EXAMPLE II

Where economic considerations control, or where properties other thanstrength of the battery grids are to be maximized, a suitable grid alloymay consist essentially of the following: 0.04 wt-% calcium, 0.08 wt-%strontium, 0.5% tin, and trace impurities not exceeding about 0.02 wt-%,with the remainder being lead.

EXAMPLE III

If desired, trace amounts of particular elements may be utilized inorder to impart other characteristics to such grid alloys, as forexample the addition of aluminum to the alloy as an oxidation inhibitor,etc., provided those trace elements do not exceed 0.03 wt-% and arepreferably held to about 0.02 wt-% or less. Accordingly, in thisembodiment a suitable alloy may consist essentially of 0.01 to 0.09 wt-%calcium, preferably 0.04 wt-%; 0.01 to 0.10 wt-% strontium, preferably0.04 wt-%; 0.01 to 1.0 wt-% tin, preferably 0.3 to 0.5 wt-%; and traceelements not to exceed 0.03 wt-%, preferably 0.02 wt-% of, for example,aluminun, with the remainder being lead.

It will be understood that various changes in the details and materialswhich have been herein described and illustrated in order to explain thenature of this invention may be made by those skilled in the art withinthe principle and scope of the invention as expressed in the followingclaims.

It will further be understood that the "Abstract of the Disclosure" setforth above is intended to provide a non-legal technical statement ofthe contents of the disclosure in compliance with the Rules of Practiceof the United States Patent and Trademark Office, and is not intended tolimit the scope of the invention described and claimed herein.

What is claimed is:
 1. In a lead-acid storage battery having a plurality of grids disposed therein; the improvement wherein said grids comprise a lead alloy consisting essentially of about 0.04 wt-% calcium; about 0.04 wt-% strontium; 0.1 to 1.0 wt-% tin; trace impurities not exceeding 0.03 wt-%, with the remainder of said alloy being lead.
 2. The invention of claim 1 wherein said tin is present in the amount of between 0.3 to 0.5 wt-%. 